This invention relates to a fuel assembly, and more particularly to a fuel assembly which can be loaded suitably into a boiling water reactor.
A conventional fuel assembly loaded into a boiling water reactor comprises a channel box having a square tubular form and a fuel bundle disposed inside the channel box. This fuel bundle consists of an upper tie plate for fitting the channel box, a lower tie plate inserted into the lower end portion of the channel box, a plurality of spacers, disposed inside the channel box and spaced from one another in an axial direction, and a plurality of fuel rods and water rods each having their both end portions fixed to the upper and lower tie plates while passing through the spacers. In this fuel assembly, cooling water under a slightly unsaturated state flows among the fuel rods through holes of the lower tie plate. While it flows from the lower portion to the upper portion among the fuel rods, the cooling water is heated, boiled to a two-phase flow and flows out of holes of the upper tie plate. The void fraction at the outlet of the fuel assembly is about 70%, and the upper portion of the fuel assembly is under a state short of light water as a neutron moderator.
Control rods and neutron detector instrumentation pipes are interposed among the fuel assemblies. Accordingly, the space among the fuel assemblies must be wide enough to dispose them, and fuel rods disposed around the peripheral portion of each fuel assembly are encompassed by a greater quantity of water than the fuel rods that are disposed at the center.
As described above, the cooling water in the fuel assembly under the state of operation has two-phase flow portions and the portions of saturated water portions distributed unevenly in both horizontal and vertical directions. The nuclear characteristics of the fuel assembly depend upon the ratio of the moderator to the fuel, and the nuclear characteristics of the conventional fuel assembly remarkably vary depending upon the position.
As a structure which makes the moderator/fuel ratio at each position of the fuel assembly close to an optimal value and improves such a nonhomogeneous moderator effect, water rods are used. In addition, a structure which further improves a moderator effect by the water rods described above is disclosed in Japanese Patent Laid-Open No. 50498/1977. In this prior art reference, a path for the moderator (two-phase flow) which expands progressively upward is disposed inside the fuel assembly, or moderator rods arranged so that the total cross-section expands progressively upward are provided.
Among the conventional methods described above, those which use the water rods or the moderator rods do not take into consideration increasing of a necessary fuel enrichment or shortening of fuel life due to a decrease of fuel loaded according to loading of the water rods or the moderator rods, and have low economical efficiency of fuel. In case a necessary amount of fuel to be loaded is maintained by increasing the diameter of the fuel rods from the aspect of fuel cycle economy, however, any consideration is not made to the decrease of the area of a coolant flow path and the increase in a wetted perimeter, so that the pressure drop in the fuel assembly increases.